This invention relates to the field of radio communications, and more specifically to spiral or helical antennas for use in wireless communication devices and systems.
Small, low profile, circular polarized (CP) antennas are used in the mobile communication industry, usually for satellite communication. As the demand for mobile handsets increases, there is a growing need for antennas of this type, and especially for low cost GPS antennas.
One solution to providing a low profile CP antenna is a patch or microstrip antenna. In order to achieve circular polarization, patch antennas need to be a half wavelength long. A patch antenna""s free-space half wavelength is usually too long for the compact space that is provided within a wireless communications device, of which a mobile handset is an example. As a result, the physical size of such a patch antenna must be reduced dramatically, using ceramics having a high dielectric constant. However, the use of ceramics having a high dielectric constant increases antenna cost, and also reduces the efficiency of the patch antenna.
FIG. 1A shows a standard-technology, dielectric-loaded, ceramic-body, hexagonal patch antenna 10 that is tuned to the global positioning system (GPS) frequency (1575.42 MHz, referred to as L1) wherein a high dielectric constant (er=40) ceramic body portion 11 was used to reduce the physical size of patch antenna 10 to less than one inch, which size is usually desirable for mobile wireless communication applications.
FIG. 1B shows the frequency/magnitude characteristic of antenna 10, wherein antenna 10 included a ceramic body portion 11, a top-located metal radiating/receiving surface 12 that lies in the X-Y plane of FIG. 1B, an off-center feed conductor 13 that extends in the Z-direction in FIG. 1B and was connected to a metal-plated top surface 12, a vertex-to-vertex dimension 14 of about 0.88 inch, and a flat-to-flat dimension 15 of about 0.72 inch.
For wireless communications systems that can tolerate relatively large antennas, the following CP antennas are standard solutions: (1) single helix antennas which have a single feed and are typically a few wavelengths tall, (2) multi-filar helix antennas that have a 90 degree hybrid and are that are typically a few wavelengths tall, (3) crossed dipole antennas that have a 90-degree hybrid and are typically a quarter wavelength tall over a ground plane, or (4) spiral antennas that have a single balanced feed and are typically a quarter wavelength tall over a ground plane.
This invention provides a small, low-profile, tri-filar helix antenna, which can have either linear polarization or CP, the antenna being provided with a single feed, and the antenna having no internal feed network.
Antennas in accordance with the invention include three metallic, bent, quarter wave monopoles, wherein only one of the monopoles is fed, and wherein the other two monopoles are parasitically coupled to the fed-monopole.
The three bent monopoles of the invention are physically positioned at 0, 120, and 240 degrees, respectively. The three monopoles are self-supporting, or they are supported on a relatively flat dielectric surface. Only one of the three monopoles is fed, for example using an inductive shunt match. The other two monopoles are strongly coupled to, and parasitically feed from, the directly-fed monopole. The two parasitic monopoles are fed at phases that are controlled by the incorporation of, or by the non-incorporation of metal perturbations within the two parasitic monopoles.
In order to induce linear polarization, no metal perturbations are used within the two parasitic monopoles, and the two parasitic monopoles are coupled at positive 120 degrees to the directly-fed monopole.
In order to induce CP, one of the two parasitic monopoles couples at positive 120 degrees to the directly-fed monopole, and the other parasitic monopole couples at negative 120 degrees to the directly-fed monopole. A metal perturbation on a given parasitic monopole operates to offset the resonant frequency of that parasitic monopole, which in turn affects the phase of coupling of that parasitic monopole to the fed-monopole.
Various metal perturbation options are available in order to generate the phase of this coupling to the directly-fed monopole. One of the parasitic monopoles can have a capacitive shunt, and the other parasitic monopole can have a series inductance, or only one parasitic monopole can have a metal perturbation, either a capacitive perturbation or an inductive perturbation, depending on the sense of the CP that is desired.
The three monopoles in accordance with the invention can be physically supported by a dielectric substrate member, or the three monopoles can be constructed of a material that renders the monopoles free-standing. A metallic ground plane is desirable directly under the three monopoles.
Antennas in accordance with the invention find utility as replacements for a dielectrically-loaded, single feed, CP patch antenna.
Antennas in accordance with the invention do not require dielectric loading. Hence, antennas in accordance with the invention are a less expensive choice for narrow band CP applications.
Antennas in accordance with an embodiment of the invention include three bent quarter wave monopoles, wherein only one of the bent monopoles is fed, and wherein the other two bent monopoles are parasitically coupled to the fed-monopole.
The bent monopoles were, for example, physically positioned at 0, 120, and 240 degrees, respectively. Only one of the bent monopoles was fed, for example with an inductive shunt match. The other two bent monopoles were excited parasitically from the fed-monopole with phases that were controlled by the incorporation of, or by the non-incorporation of, perturbations on or within the two parasitically-fed monopoles.
In antennas constructed and arranged in accordance with the invention the magnitude of the above-described parasitic coupling was relatively large (for example about xe2x88x926 dB), and this relatively large parasitic coupling between the directly excited monopole and the two parasitic monopoles provided that the antenna generated a symmetric radiation pattern. This relatively large parasitic coupling also effectively acts as a feed network to the two parasitically coupled monopoles, and allows the antenna to have just one of the monopoles directly excited. This relatively large parasitic coupling is, to a large extent, controlled by the width of a capacitive gap that existed between the two parasitic monopoles and the fed-monopole.
In summary, the present invention provides a small, low-profile, single feed, linear polarized or CP, tri-filar, helix antenna having three bent quarter wave monopoles that are physically positioned at about 0, 120, and 240 degrees, respectively. The outer perimeter of the antenna can be a hexagon, or it can be circular, it can approach a circular shape, or it can have a number of sides equal to 6xc3x97N where N is an integer that is greater then zero.
Linear antenna polarization is produced when no perturbations are provided for either of the two parasitic monopoles, in which case both of the parasitic monopoles are excited parasitically in-phase at positive 120 degrees.
In order to produce CP, metal perturbations are applied to the two parasitic monopoles in order to generate a positive 120 degree parasitic coupling in one of the parasitic monopoles, and to in order to generate a negative 120 degree parasitic coupling in the other of the two parasitic monopoles.
Various perturbation options can be used to generate the above phasing. For example, one of the two parasitic monopoles can include a capacitive shunt, and the other parasitic monopole can include an inductive shunt. Or, only one of the two parasitic monopoles can be provided with a perturbation, either a capacitive perturbation or an inductive perturbation, depending on the sense of the CP that is desired.
The reactive-capacitance or reactive-inductance perturbations can be provided either by shaping the metal legs of the parasitic monopoles, or by connecting discrete capacitive or inductive electrical components to the parasitic monopoles.
With only one monopole directly fed, the large coupling between this directly-fed monopole and the two parasitic monopoles acts as a feed network to the two parasitic monopoles. It is desirable that all three monopoles be fed with equal RF energy levels in their resonant condition, such that the three monopole antenna will generate three symmetric radiation patterns.
In practice it is desirable that one half of the RF energy that is provided as an input to the directly-fed monopole be coupled to the two parasitic monopoles, and that the other half of this RF energy be radiated into free space.
If coupling from the directly-fed monopole to the two parasitic monopoles is significantly larger than this one-half amount, each of the three monopoles may act as a poor radiator, and the efficiency of the three monopole antenna may be reduced. If the coupling from the directly-fed monopole to the two parasitic monopoles is significantly smaller than this one-half amount it may be difficult to parasitically excite the two parasitic monopoles in order to generate CP.